1. Field of the Invention
The present invention relates to lithographic apparatus and methods.
2. Description of the Related Art
In order to determine a position of an object very accurately, an object position measuring system, such as a laser interferometer system, may be used. In a laser interferometer system, which is an incremental system where the wavelength of the laser light used constitutes a measurement unit, a reflection of a laser beam against a reflecting surface of the object is measured and compared with an internal reference path. If the object position changes in line with the laser beam, the comparison between the reference and the measuring laser beam will show a number of interference transitions (consisting of constructive/destructive interference between the measuring path and the reference path) which is proportional to the displacement. The number of interference transitions is not only proportional to the displacement, but also with the wavelength of the laser light. In particular, the number of interference transitions is equal to the change of length of the optical path as a result of the displacement, divided by the wavelength.
The wavelength of light in air depends on the nominal wavelength in vacuum, divided by the refractive index of the medium through which the light travels. The refractive index in air nair may be described by the so-called Edlen formula (B. Edlen, The refractive index of air, Metrologia, Vol. 2, nr. 2, pp. 71-80 (1966), included herein by reference):
                              n          air                =                                            D              ·              0.104126              ·                              10                                  -                  4                                            ·              P                                      1              +                              0.003671                ·                T                                              -                      0.42066            ·                          10                              -                9                                      ·            F                    +          1                                    (        1        )            wherein:    D=0.27651754·10−3(1+53.5·10−8(C-300))    P corresponds to the pressure (of air) [Pa]    T corresponds to the temperature (of air) [° C.]    F corresponds to the partial water vapor pressure [Pa], and    C corresponds to the CO2 concentration [ppm]
Another version of formula (1), in which the basic interdependencies of the parameters are unchanged, is described in K. P. Birch, M. J. Downs, 1994, Correction of the updated Edlen equation for the refractive index of air, Metrologia 31, pp. 315-316 (included herein by reference).
It will be appreciated that nair, in formula (1), depends on the pressure of the air as well as on other parameters which are not considered in more detail in the context of the present invention.
Consequently, when measuring a position with a laser interferometer system, it is desirable to take into account the pressure of the air. When the pressure is accurately known, the refractive index may be accurately determined. Further, the number of interference transitions (“fringe count”) may be accurately determined in the laser interferometer system. Combining the results of the pressure determination and the fringe count, the position sought may be accurately determined.
It results from the foregoing discussion that, in order to accurately measure an object position in a medium, such as air, using a laser interferometer system or any other measurement system based on the determination of a number of wavelengths, it is desirable to accurately determine the pressure of the medium.
A lithographic apparatus is an example of a tool, in which high accuracy of position measurement is desired. A lithographic apparatus generally employs a laser interferometer measurement system.
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the mask and the substrate (or their respective stages, commonly referred to as reticle stage (or mask table or patterning support) and wafer stage (or substrate table or substrate support)) should be positioned accurately. For this purpose, a laser interferometer system may be used. In an initialization phase, the accuracy of the alignment may be limited, since possible position offsets may be corrected by using alignment sensors for scaling on alignment marks previously provided on the mask and the substrate, or on the carriers thereof. After the initialization phase, when the alignment position is left, the alignment accuracy is generally high.
In view of the influence of the pressure on the actual wavelength, as explained above, in the initialization phase, the absolute pressure may be measured with a limited (absolute) accuracy, and be used to determine the actual wavelength with respect to the known value in vacuum, e.g. through formula (1) or its other version. Since, in the initialization phase, a scaling is performed using the alignment marks, the accuracy and the repeatability of the pressure measurement may be relatively low.
However, after the alignment of the reticle stage or wafer stage, pressure changes may directly influence the stage positioning. The scaling effect of a pressure change on the wavelength measured by the laser interferometer system is proportional to the actual pressure change over the absolute pressure.
It may be difficult with conventional pressure sensors to measure absolute pressure with a high accuracy, and to measure fast pressure changes with sufficient speed and accuracy.